The present invention relates to complete knee prosthesis. Such prosthesis comprises, as it is well known, a femoral element reproducing the femoral condyles, a tibial seat forming an upper support plate and a medial plateau made of a material promoting a slipping movement, notably of high density polyethylene. This medial plateau has a lower face intended for resting against the upper support plate of the tibial seat and glenoid cavities receiving the condyles of the femoral element.
In a complete knee prosthesis, it is advisable that the prosthetic condyles reproduce globally the form of the natural condyles of the knee joint, to obtain good reproduction of the movement of this joint and of an adequate ligamental tension. These prosthetic condyles must therefore have, as seen in the sagittal plane, a spiral form corresponding substantially to that of the natural condyles, i.e. with gradual reduction of their radii towards the posterior sectors.
There results from these reduced radii that the contact surface between the condyles and the medial plateau varies with the flexion movement or the extension of the joint.
Certain prostheses promote the congruence in extension, i.e. between 0 and 40° to 60° approx. in flexion. In such a case, the contact of the prosthetic condyles with the medial plateau, beyond these angles of flexion, becomes linear or even punctual, and non surface-related, which enables the femur to move forward with respect to the tibia, according to a so-called “posterior drawer” movement. This movement is contrary to the physiologic recall movement of the femur with respect to the tibia during the so-called “roll-back” flexion.
It is advisable to reproduce this “roll-back” movement on a knee prosthesis. Indeed, this movement enables to increase the extension force of the quadriceps, while increasing the lever arm thereof. Moreover, this movement enables to delay the moment when the femur abuts against the medial plateau at the end of the flexion and to reduce the camming effect of the soft portions in this very position, which increases the amplitude of movement of the joint and limits the stresses on the bony anchoring interfaces of the femoral element and of the tibial seat.
To solve the problem of so-called “posterior drawer” movement, it is known to adjoin to a knee prosthesis a “postero-stabilisation” system, which consists of a stop between a bar or a lobe provided on the femoral element, between both condyles, and a medial prominence of the medial plateau. This system limits the forward movement of the femur with respect to the tibia, but the contact surface between the femur and the medial element is necessarily linear or punctual in flexion. This limited contact surface is a source of wear for the medial plateau and is therefore detrimental to the perennity thereof.
Certain prostheses enable surface congruence in extension and flexion: the posterior sagittal condylian radius inscribes a circle, and the medial plateau has glenoid surfaces with corresponding circular profile. In such a case, the surface contact is maintained throughout the flexion, but, taking into account this congruence, the translation movements of the femur with respect to the medial plateau are limited or impossible.
Certain prostheses enable antero-posterior mobility between the medial plateau and the tibial element. In such a case, the movement of the femur with respect to the tibia results not from the form of the contact surfaces but from the action of the muscles and of the ligaments. The “roll-back” movement aforementioned being essentially controlled by the posterior cross ligament, these prostheses impose the conservation of this posterior cross ligament, which is not always possible, requested or advisable.